Rubber composition for tires and pneumatic tire

ABSTRACT

A rubber composition for tires of the present technology contains a diene rubber, silica, and a predetermined alkyltrialkoxysilane; the diene rubber containing a butadiene rubber and a particular conjugated diene rubber, a content of the butadiene rubber in the diene rubber being 20 mass % or greater and a content of the particular conjugated diene rubber in the diene rubber being from 30 to 80 mass %; an average glass transition temperature of the diene rubber being from −65 to −45° C.; the particular conjugated diene rubber being a conjugated diene rubber produced by a particular production method and having predetermined ranges of aromatic vinyl unit content, vinyl bond content, and weight average molecular weight.

TECHNICAL FIELD

The present technology relates to a rubber composition for tires and apneumatic tire.

BACKGROUND ART

For winter tires for use on icy and snowy road surfaces, performance onicy and snowy roads—specifically, superior braking ability on icy andsnowy roads—is demanded in addition to dry grip performance, wet gripperformance, and wear resistance.

To satisfy such a demand, for example, Japanese Unexamined PatentApplication Publication No. 2010-270207A proposes “a rubber compositionfor a tire tread including a diene rubber component and silica andcarbon black, the diene rubber component including: (A) from 30 to 80mass % of a hydroxy group-containing aromatic vinyl-conjugated dienecopolymer containing from 20 to 30 mass % of aromatic vinyl units andfrom 0.1 to 10 mass % of isoprene units, and having an amount of vinylbonds in a conjugated diene part of 40 to 60 mol %; (B) from 10 to 50mass % of a high-cis butadiene rubber having 90 mol % or greater ofcis-1,4 bond content; and (C) from 10 to 50 mass % of natural rubber; atotal amount of silica and carbon black being from 90 to 150 parts bymass per 100 parts by mass of the diene rubber component” (claim 1), andalso describes a winter tire including this rubber composition for atread (claim 5 and claim 6).

When the inventors of the present technology studied the rubbercomposition for tire treads described in Japanese Unexamined PatentApplication Publication No. 2010-270207A, the inventors found that thewet grip performance (hereinafter, simply abbreviated as “wetperformance”) and/or performance on icy and snowy roads of the producedtire may be poor depending on the type and compounded amount of dienerubber.

SUMMARY

The present technology provides a rubber composition for tiresexhibiting excellent wet performance and performance on icy and snowyroads when the rubber composition is formed into a tire, and a pneumatictire including the rubber composition for tires.

By blending a specific amount of predetermined conjugated diene rubberand blending specific amounts of silica and a predeterminedalkyltrialkoxysilane, excellent wet performance and excellentperformance on icy and snowy roads can be achieved when a tire isproduced.

[1] A rubber composition for tires including a diene rubber, silica, andan alkyltrialkoxysilane represented by Formula (I);

the diene rubber containing a butadiene rubber and a particularconjugated diene rubber, a content of the butadiene rubber in the dienerubber being 20 mass % or greater and a content of the particularconjugated diene rubber in the diene rubber being from 30 to 80 mass %;

an average glass transition temperature of the diene rubber being from−65 to −45° C.;

a content of the silica being from 90 to 150 parts by mass per 100 partsby mass of the diene rubber;

a content of the alkyltrialkoxysilane being from 0.1 to 8 mass %relative to the content of the silica;

wherein, R¹¹ represents an alkyl group having from 1 to 20 carbons, andR¹² each independently represents a methyl group or an ethyl group;

the particular conjugated diene rubber being a conjugated diene rubberproduced by a method of producing a conjugated diene rubber, the methodincluding steps A, B, and C in this order;

the particular conjugated diene rubber having an aromatic vinyl unitcontent of 38 to 48 mass %, a vinyl bond content of 20 to 35 mass %, anda weight average molecular weight of 500000 to 800000.

Step A: a step of forming a polymer block A having an active terminal,the polymer block A having an isoprene unit content of 80 to 95 mass %,an aromatic vinyl unit content of 5 to 20 mass %, and a weight averagemolecular weight of 500 to 15000, by polymerizing a monomer mixturecontaining isoprene and an aromatic vinyl.

Step B: a step of obtaining a conjugated diene-based polymer chainhaving an active terminal, the conjugated diene-based polymer chainhaving the polymer block A and a polymer block B, by forming the polymerblock B having an active terminal by mixing the polymer block A with amonomer mixture containing 1,3-butadiene and an aromatic vinyl tocontinue polymerization reaction to form the polymer block B in serieswith the polymer block A.

Step C: a step of reacting a polyorganosiloxane represented by Formula(1) with the active terminal of the conjugated diene-based polymerchain.

In Formula (1), R₁ to R₈ are the same or different and are each an alkylgroup having from 1 to 6 carbons or an aryl group having from 6 to 12carbons. X₁ and X₄ are the same or different and are groups selectedfrom the group consisting of alkyl groups having from 1 to 6 carbons,aryl groups having from 6 to 12 carbons, alkoxy groups having from 1 to5 carbons, and groups having an epoxy group and from 4 to 12 carbons. X₂is an alkoxy group having from 1 to 5 carbons or a group having an epoxygroup and from 4 to 12 carbons, and a plurality of the X₂ moieties arethe same or different. X₃ is a group having from 2 to 20 alkylene glycolrepeating units, and when a plurality of the X₃ moieties exists, the X₃moieties are the same or different. m is an integer of 3 to 200, n is aninteger of 0 to 200, and k is an integer of 0 to 200.

[2] The rubber composition for tires according to [1], further includinga silane coupling agent, the silane coupling agent and the silicasatisfying Equation (A). Equation (A): [content of silane coupling agent(g)×100/{content of silica (g)×CTAB (cetyltrimethylammonium bromide)adsorption specific surface area (m²/g)}]×100=3.5 to 7.5

[3] The rubber composition for tires according to [1] or [2], where thesilica contains 50 mass % or greater of silica (X) having a CTABadsorption specific surface area of 80 to 130 m²/g.

[4] The rubber composition for tires according to any one of [1] to [3],further including an aromatic modified terpene resin having a softeningpoint of 100 to 150° C.,

a content of the aromatic modified terpene resin being from 3 to 20parts by mass per 100 parts by mass of the diene rubber.

[5] A pneumatic tire having the rubber composition for tires describedin any one of [1] to [4] in a cap tread.

[6] The pneumatic tire according to [5], the pneumatic tire being usedfor a winter tire.

As described below, according to the present technology, a rubbercomposition for tires exhibiting excellent wet performance andperformance on icy and snowy roads when the rubber composition is formedinto a tire, and a pneumatic tire including the rubber composition fortires can be provided.

Note that, hereinafter, achieving excellent wet performance when a tireis produced is simply abbreviated as “achieving excellent wetperformance”, and achieving excellent performance on icy and snowy roadswhen a tire is produced is simply abbreviated as “achieving excellentperformance on icy and snowy roads”.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a partial cross-sectional schematic view of a tire thatillustrates one embodiment of a pneumatic tire of the presenttechnology.

DETAILED DESCRIPTION

Hereinafter, a rubber composition for tires of the present technologyand a pneumatic tire including the rubber composition for tires of thepresent technology will be described.

In this specification, a numerical range represented using “(from) . . .to . . . ” refers to a range including a numerical values preceding “to”as a lower limit value and a numerical value following “to” as an upperlimit value, respectively.

Rubber Composition for Tires

The rubber composition for tires of the present technology (hereinafter,simply abbreviated as “rubber composition of the present technology”)contains a diene rubber, silica, and an alkyltrialkoxysilane representedby Formula (I) described below.

Note that the diene rubber contains a butadiene rubber and a particularconjugated diene rubber. The content of the butadiene rubber in thediene rubber is 20 mass % or greater and the content of the particularconjugated diene rubber in the diene rubber is from 30 to 80 mass %.Furthermore, the average glass transition temperature of the dienerubber is from −65 to −45° C.

Furthermore, the particular conjugated diene rubber is a conjugateddiene rubber produced by a method of producing a conjugated dienerubber, the method including steps A, B, and C described below in thisorder. The conjugated diene rubber has an aromatic vinyl unit content of38 to 48 mass %, a vinyl bond content of 20 to 35 mass %, and a weightaverage molecular weight of 500000 to 800000.

Furthermore, the content of the silica is from 90 to 150 parts by massper 100 parts by mass of the diene rubber.

Similarly, the content of the alkyltrialkoxysilane is from 0.1 to 8 mass% relative to the content of the silica.

Since the rubber composition of the present technology has such aconfiguration, both wet performance and performance on icy and snowyroads become excellent when the rubber composition is formed into atire. Although the reason for this is unknown, the reason is presumed tobe as follows.

It is known that characteristics such as wet performance are enhanced byblending silica and/or an alkoxysilane; however, it is also known thatprocessability or the like is deteriorated due to aggregation since thesilica tends to aggregate.

Note that the particular conjugated diene rubber contained in the rubbercomposition of the present technology is obtained by forming arubber-based polymer block B to a polymer block A, which is formed bypolymerizing a monomer mixture containing isoprene and an aromaticvinyl, and then reacting a particular polyorganosiloxane.

Therefore, in the present technology, by using a diene rubber containingpredetermined amounts of a butadiene rubber and a particular conjugateddiene rubber, silica can be highly dispersed due to strong affinity ofthe polyorganosiloxane in the particular conjugated diene rubber withthe silica in the composition, while low-temperature properties of thebutadiene rubber is maintained. It is conceived that both the wetperformance and the performance on icy and snowy roads become excellentas a result.

Each of the components contained in the rubber composition of thepresent technology will be described in detail below.

Diene Rubber

The diene rubber contained in the rubber composition of the presenttechnology contains a butadiene rubber and a particular conjugated dienerubber.

Butadiene Rubber

The butadiene rubber contained in the diene rubber is not particularlylimited.

The content of the butadiene rubber in the diene rubber is 20 mass % orgreater, and preferably from 20 to 50 mass %.

Note that “content of the butadiene rubber in the diene rubber”indicates the content (mass %) of the butadiene rubber relative to thetotal amount of the diene rubber.

In the present technology, the butadiene rubber is preferably abutadiene rubber with a high cis structure, and is specifically abutadiene rubber with a cis-1,4 bond content of 90% or greater, andpreferably 95% or greater.

Note that such a butadiene rubber with a high cis structure can bepolymerized by a typical method using a Ziegler catalyst, neodymiumcatalyst, or the like.

The weight average molecular weight of the butadiene rubber ispreferably from 50000 to 1000000, and more preferably from 200000 to800000.

Note that the weight average molecular weight (Mw) of the butadienerubber is measured by gel permeation chromatography (GPC) based oncalibration with polystyrene standard using tetrahydrofuran as asolvent.

Particular Conjugated Diene Rubber

As described above, the particular conjugated diene rubber is aconjugated diene rubber produced by a method of producing a conjugateddiene rubber, the method including steps A, B, and C described below inthis order. The conjugated diene rubber has an aromatic vinyl unitcontent of 38 to 48 mass %, a vinyl bond content of 20 to 35 mass %, anda weight average molecular weight of 500000 to 800000.

Step A: a step of forming a polymer block A having an active terminal,the polymer block A having an isoprene unit content of 80 to 95 mass %,an aromatic vinyl unit content of 5 to 20 mass %, and a weight averagemolecular weight of 500 to 15000, by polymerizing a monomer mixturecontaining isoprene and an aromatic vinyl.

Step B: a step of obtaining a conjugated diene-based polymer chainhaving an active terminal, the conjugated diene-based polymer chainhaving the polymer block A and a polymer block B, by forming the polymerblock B having an active terminal by mixing the polymer block A with amonomer mixture containing 1,3-butadiene and an aromatic vinyl tocontinue polymerization reaction to form the polymer block B in serieswith the polymer block A.

Step C: a step of reacting a polyorganosiloxane represented by Formula(1) below with the active terminal of the conjugated diene-based polymerchain.

Each of the steps will be described in detail below.

Step A

In Step A, a polymer block A having an active terminal and having anisoprene unit content of 80 to 95 mass %, an aromatic vinyl unit contentof 5 to 20 mass %, and a weight average molecular weight of 500 to15000, is formed by polymerizing a monomer mixture containing isopreneand an aromatic vinyl.

The monomer mixture may only contain isoprene and an aromatic vinyl, andmay contain another monomer besides the isoprene and the aromatic vinyl.

The aromatic vinyl is not particularly limited; however, examplesthereof include styrene, α-methyl styrene, 2-methyl styrene, 3-methylstyrene, 4-methyl styrene, 2-ethylstyrene, 3-ethyl styrene, 4-ethylstyrene, 2,4-diisopropylstyrene, 2,4-dimethyl styrene, 4-t-butylstyrene, 5-t-butyl-2-methylstyrene, vinylnaphthalene,dimethylaminomethylstyrene, dimethylaminoethylstyrene, and the like.Among these, styrene is preferred. A single aromatic vinyl may be usedalone or a combination of two or more types of these aromatic vinyls maybe used.

Examples except the aromatic vinyl of the monomer, excluding theisoprene and the aromatic vinyl, include conjugated diene exceptisoprene, such as 1,3-butadiene, 2,3-dimethyl-1,3-butadiene,2-chloro-1,3-butadiene, 1,3-pentadiene, and 1,3-hexadiene; α- andβ-unsaturated nitriles, such as acrylonitrile and methacrylonitrile;unsaturated carboxylic acids or acid anhydrides, such as acrylic acid,methacrylic acid, and maleic anhydride; unsaturated carboxylates, suchas methylmethacrylate, ethylacrylate, and butylacrylate; andnon-conjugated dienes, such as 1,5-hexadiene, 1,6-heptadiene,1,7-octadiene, dicyclopentadiene, and 5-ethylidene-2-norbornene. Amongthese, 1,3-butadiene is preferred. A single type of these may be usedalone or a combination of two or more types of these may be used.

The monomer mixture described above is preferably polymerized in aninert solvent.

The inert solvent is not particularly limited as long as the inertsolvent is an inert solvent typically used in solution polymerizationand does not hinder the polymerization reaction. Specific examplesthereof include chain aliphatic hydrocarbons, such as butane, pentane,hexane, heptane, and 2-butene; alicyclic hydrocarbons, such ascyclopentane, cyclohexane, and cyclohexene; aromatic hydrocarbons, suchas benzene, toluene, and xylene. The used amount of the inert solventis, for example, an amount such that the monomer mixture concentrationis from 1 to 80 mass %, and preferably from 10 to 50 mass %.

The monomer mixture described above is preferably polymerized by apolymerization initiator.

The polymerization initiator is not particularly limited as long as thepolymerization initiator can polymerize the monomer mixture containingisoprene and an aromatic vinyl and can form a polymer chain having anactive terminal. As the specific examples thereof, an organoalkali metalcompound and organoalkaline earth metal compound as well as apolymerization initiator having a lanthanide series metal compound orthe like as a primary catalyst is preferably used. Examples of theorganoalkali metal compound include organomonolithium compounds, such asn-butyllithium, sec-butyllithium, t-butyllithium, hexyllithium,phenyllithium, and stilbene lithium; organopolylithium compounds, suchas dilithiomethane, 1,4-dilithiobutane, 1,4-dilithio-2-ethylcyclohexane,1,3,5-trilithiobenzene, and 1,3,5-tris(lithiomethyl)benzene;organosodium compounds, such as sodium naphthalene; and organopotassiumcompounds, such as potassium naphthalene. Examples of the organoalkalineearth metal compound include di-n-butylmagnesium, di-n-hexylmagnesium,diethoxycalcium, calcium distearate, di-t-butoxystrontium,diethoxybarium, diisopropoxybarium, diethylmercaptobarium,di-t-butoxybarium, diphenoxybarium, diethylaminobarium, bariumdistearate, and diketylbarium. Examples of the polymerization initiatorhaving a lanthanide series metal compound as a primary catalyst includepolymerization initiators containing a primary catalyst of a lanthanideseries metal salt containing a lanthanide series metal such aslanthanum, cerium, praseodymium, neodymium, samarium, and gadolinium,and a carboxylic acid or phosphorus-containing organic acid, togetherwith a co-catalyst such as an alkylaluminum compound, organoaluminumhydride compound, or organoaluminum halide compound. Among thesepolymerization initiators, an organomonolithium compound is preferablyused, and n-butyllithium is more preferably used. Furthermore, theorganoalkali metal compound may be used as an organoalkali metal amidecompound after being subjected to a reaction with a secondary amine,such as dibutylamine, dihexylamine, dibenzylamine, pyrrolidine,hexamethyleneimine, and heptamethyleneimine. A single polymerizationinitiator may be used alone or a combination of two or morepolymerization initiators may be used.

The used amount of the polymerization initiator may be selecteddepending on the targeted molecular weight, but the used amount ispreferably from 4 to 250 mmol, more preferably from 6 to 200 mmol, andparticularly preferably from 10 to 70 mmol, per 100 g of the monomermixture.

The polymerization temperature at which the monomer mixture ispolymerized is, for example, in a range from −80 to +150° C., preferablyfrom 0 to 100° C., and more preferably from 20 to 90° C.

The polymerization mode may be some modes such as batch mode orcontinuous mode. The bonding type may be a variety of bonding types suchas block, tapered, random, or the like.

Examples of the method of adjusting the 1,4-bond content in the isopreneunit in the polymer block A include a method in which a polar compoundis added in an inert solvent during the polymerization and the addedamount thereof is adjusted. Examples of the polar compound include ethercompounds, such as dibutyl ether, tetrahydrofuran, and2,2-di(tetrahydrofuryl)propane; tertiary amines, such astetramethylethylenediamine; alkali metal alkoxides; and phosphinecompounds. In particular, ether compounds and tertiary amines arepreferred. Among these, those capable of forming a chelate structurewith the metal of the polymerization initiator are more preferred, and2,2-di(tetrahydrofuryl)propane and tetramethylethylenediamine areparticularly preferred.

The used amount of the polar compound may be selected depending on thetargeted 1,4-bond content, and is preferably from 0.01 to 30 mol, andmore preferably from 0.05 to 10 mol, per 1 mol of the polymerizationinitiator. When the used amount of polar compound is within the rangedescribed above, it is easy to adjust the 1,4-bond content in theisoprene unit, and problems due to deactivation of the polymerizationinitiator are less likely to occur.

The 1,4-bond content in the isoprene unit in the polymer block A ispreferably from 10 to 95 mass %, and more preferably from 20 to 95 mass%.

Note that, in the present specification, “1,4-bond content in theisoprene unit” indicates the proportion (mass %) of the isoprene unitwith a 1,4-bond relative to the total amount of the isoprene unitcontained in the polymer block A.

The weight average molecular weight (Mw) of the polymer block A measuredby gel permeation chromatography (GPC) based on calibration withpolystyrene is from 500 to 15000. In particular, the weight averagemolecular weight (Mw) is preferably from 1000 to 12000, and morepreferably from 1500 to 10000.

When the weight average molecular weight of the polymer block A is lessthan 500, desired low heat build-up and wet performance are less likelyto be exhibited.

When the weight average molecular weight of the polymer block A isgreater than 15000, the desired balance of viscoelastic characteristicswhich serves as an indicator of low rolling and wet performance may bedeteriorated.

The weight average molecular distribution expressed by the ratio (Mw/Mn)of the weight average molecular weight (Mw) to the number averagemolecular weight (Mn) of the polymer block A is preferably from 1.0 to1.5, and more preferably from 1.0 to 1.3. When the value (Mw/Mn) of themolecular weight distribution of the polymer block A is within the rangedescribed above, the production of the particular conjugated dienerubber is more facilitated. Note that both Mw and Mn are values measuredby GPC based on calibration with polystyrene.

The isoprene unit content of the polymer block A is from 80 to 95 mass%, and preferably from 85 to 95 mass %.

The aromatic vinyl content of the polymer block A is from 5 to 20 mass%, preferably from 5 to 15 mass %, and more preferably from 5 to 13 mass%.

The content of the monomer unit except the isoprene and the aromaticvinyl in the polymer block A is preferably 15 mass % or less, morepreferably 10 mass % or less, and even more preferably 6 mass % or less.

Step B

In Step B, a conjugated diene-based polymer chain having an activeterminal and having the polymer block A and a polymer block B isobtained by forming the polymer block B having an active terminal bymixing the polymer block A formed in Step A described above with amonomer mixture containing 1,3-butadiene and an aromatic vinyl tocontinue polymerization reaction to form the polymer block B in serieswith the polymer block A.

Specific examples and preferred aspects of the aromatic vinyl are asdescribed above.

The monomer mixture described above is preferably polymerized in aninert solvent.

The definition, specific examples, and preferred aspects of the inertsolvent are as described above.

The used amount of the polymer block A having an active terminal duringthe formation of the polymer block B may be selected depending on thetargeted molecular weight and is, for example, from 0.1 to 5 mmol,preferably from 0.15 to 2 mmol, and more preferably from 0.2 to 1.5mmol, per 100 g of the monomer mixture containing 1,3-butadiene and anaromatic vinyl.

The method of mixing the polymer block A with the monomer mixturecontaining 1,3-butadiene and an aromatic vinyl is not particularlylimited. The polymer block A having an active terminal may be added to asolution of the monomer mixture containing 1,3-butadiene and an aromaticvinyl, or the monomer mixture containing 1,3-butadiene and an aromaticvinyl may be added to a solution of the polymer block A having an activeterminal. From the perspective of controlling the polymerization, addingthe polymer block A having an active terminal to a solution of themonomer mixture containing 1,3-butadiene and an aromatic vinyl ispreferred.

When the monomer mixture containing 1,3-butadiene and an aromatic vinylis polymerized, the polymerization temperature is, for example, in arange from −80 to +150° C., preferably from 0 to 100° C., and morepreferably from 20 to 90° C. The polymerization mode may be some modessuch as batch mode or continuous mode. In particular, batch mode ispreferred.

The bonding type of each monomer of the polymer block B may be a varietyof bonding types such as block, tapered, random, or the like. Amongthese, random bonding is preferred. When the bonding type between the1,3-butadiene and the aromatic vinyl is random, it is preferred that the1,3-butadiene and the aromatic vinyl are supplied and polymerizedcontinuously or intermittently to the polymerization system so that theratio of the aromatic vinyl to the total amount of the 1,3-butadiene andthe aromatic vinyl is not too high in the polymerization system.

The 1,3-butadiene unit content of the polymer block B is notparticularly limited; however, the content is preferably from 55 to 95mass %, and more preferably from 55 to 90 mass %.

The aromatic vinyl unit content of the polymer block B is notparticularly limited; however, the content is preferably from 5 to 45mass %, and more preferably from 10 to 45 mass %.

The polymer block B may further contain another monomer unit besides the1,3-butadiene unit and the aromatic vinyl unit. Examples of anothermonomer used to constitute such another monomer unit include thoseexemplified as “examples except the aromatic vinyl of the monomer,excluding the isoprene” described above except the 1,3-butadiene; andisoprene.

The content of such another monomer unit of the polymer block B ispreferably 50 mass % or less, more preferably 40 mass % or less, andeven more preferably 35 mass % or less.

To adjust the vinyl bond content in the 1,3-butadiene unit of thepolymer block B, a polar compound is preferably added to an inertsolvent in the polymerization. However, a polar compound does not haveto be added again if a polar compound has already been added to theinert solvent in an amount sufficient to adjust the vinyl bond contentin the 1,3-butadiene unit in the polymer block B when the polymer blockA is prepared. Specific examples of the polar compound used to adjustthe vinyl bond content are the same as the polar compounds used in theformation of the polymer block A described above. The used amount ofpolar compound should be determined according to the targeted vinyl bondcontent, but is preferably from 0.01 to 100 mol, and more preferablyfrom 0.1 to 30 mol, relative to 1 mol of the polymerization initiator.When the used amount of the polar compound is within this range, it iseasy to adjust the vinyl bond content in the 1,3-butadiene unit, andproblems due to deactivation of the polymerization initiator are lesslikely to occur.

The vinyl bond content in the 1,3-butadiene unit in the polymer block Bis preferably from 10 to 90 mass %, more preferably from 20 to 80 mass%, and particularly preferably from 25 to 70 mass %.

By Steps A and B, a conjugated diene-based polymer chain having anactive terminal and having the polymer block A and the polymer block Bcan be obtained.

The conjugated diene-based polymer chain having an active terminal ispreferably composed of the polymer block A-polymer block B and theterminal of the polymer block B is preferably an active terminal fromthe perspective of productivity; however, the conjugated diene-basedpolymer chain may contain a plurality of the polymer block A, or containanother polymer block. Examples thereof include conjugated diene-basedpolymer chains having an active terminal, such as polymer blockA-polymer block B-polymer block A and blocks formed only from polymerblock A-polymer block B-isoprene. When the block formed only fromisoprene is formed on the active terminal side of the conjugateddiene-based polymer chain, the used amount of isoprene is preferablyfrom 10 to 100 mol, more preferably from 15 to 70 mol, and particularlypreferably from 20 to 35 mol, per 1 mol of the polymerization initiatorused in the first polymerization reaction.

The mass ratio of the polymer block A to the polymer block B, (mass ofthe polymer block A)/(mass of the polymer block B), in the conjugateddiene-based polymer chain having an active terminal (in the case where aplurality of the polymer blocks A and B are present, based on each ofthe total mass) is preferably from 0.001 to 0.1, more preferably from0.003 to 0.07, and particularly preferably from 0.005 to 0.05.

The molecular weight distribution expressed as the ratio (Mw/Mn) of theweight average molecular weight (Mw) to the number average molecularweight (Mn) of the conjugated diene-based polymer chain having an activeterminal is preferably from 1.0 to 3.0, more preferably from 1.0 to 2.5,and particularly preferably from 1.0 to 2.2. When the value (Mw/Mn) ofthe molecular weight distribution of the conjugated diene-based polymerchain having an active terminal is within the range described above,production of the particular conjugated diene rubber is facilitated.Note that both Mw and Mn are values measured by GPC based on calibrationwith polystyrene.

In the conjugated diene-based polymer chain having an active terminal,the total content of the isoprene unit and the 1,3-butadiene unit of 50to 99.995 mass % and the content of the aromatic vinyl unit of 0.005 to50 mass % are preferred, the total content of the isoprene unit and the1,3-butadiene unit of 55 to 95 mass % and the content of the aromaticvinyl unit of 5 to 45 mass % are more preferred, and the total contentof the isoprene unit and the 1,3-butadiene unit of 55 to 90 mass % andthe content of the aromatic vinyl unit of 10 to 45 mass % areparticularly preferred. Furthermore, the vinyl bond content in theisoprene unit and the 1,3-butadiene unit in the conjugated diene-basedpolymer chain having an active terminal are similar to the vinyl bondcontent in the 1,3-butadiene unit in the polymer block B describedabove.

Step C

Step C is a step of reacting a polyorganosiloxane represented by Formula(1) below with the active terminal of the conjugated diene-based polymerchain obtained in Step B.

In Formula (1) above, R₁ to R₈ are the same or different and are each analkyl group having from 1 to 6 carbons or an aryl group having from 6 to12 carbons. X₁ and X₄ are the same or different and are groups selectedfrom the group consisting of alkyl groups having from 1 to 6 carbons,aryl groups having from 6 to 12 carbons, alkoxy groups having from 1 to5 carbons, and groups having an epoxy group and from 4 to 12 carbons. X₂is an alkoxy group having from 1 to 5 carbons or a group having an epoxygroup and from 4 to 12 carbons, and a plurality of the X₂ moieties arethe same or different. X₃ is a group having from 2 to 20 alkylene glycolrepeating units, and when a plurality of the X₃ moieties exists, the X₃moieties are the same or different. m is an integer of 3 to 200, n is aninteger of 0 to 200, and k is an integer of 0 to 200.

Examples of the alkyl groups having from 1 to 6 carbons represented byR₁ to R₈, X₁, and X₄ in the polyorganosiloxane represented by Formula(1) above include a methyl group, an ethyl group, an n-propyl group, anisopropyl group, a butyl group, a pentyl group, a hexyl group, and acyclohexyl group. Examples of the aryl groups having from 6 to 12carbons include a phenyl group, and a methylphenyl group. Among these, amethyl group and an ethyl group are preferred from the perspective ofproduction of the polyorganosiloxane itself.

Examples of the alkoxyl group having from 1 to 5 carbons represented byX₁, X₂, and X₄ in the polyorganosiloxane represented by Formula (1)above include a methoxy group, an ethoxy group, a propoxy group, anisopropoxy group, and a butoxy group. Among these, a methoxy group andan ethoxy group are preferred from the perspective of reactivity withthe active terminal of the conjugated diene-based polymer chain.

Examples of the epoxy group-containing groups having from 4 to 12carbons represented by X₁, X₂, and X₄ in the polyorganosiloxanerepresented by Formula (1) above include groups represented by Formula(2) below.*—Z₁—Z₂-E  Formula (2)

In Formula (2) above, Z₁ is an alkyl arylene group or alkylene grouphaving from 1 to 10 carbons; Z₂ is a methylene group, a sulfur atom, oran oxygen atom; and E is an epoxy group-containing hydrocarbon grouphaving from 2 to 10 carbons. In Formula (2) above, * indicates a bondposition.

In the groups represented by Formula (2) above, preferably, Z₂ is anoxygen atom; more preferably, Z₂ is an oxygen atom and E is a glycidylgroup; and particularly preferably, Z₁ is an alkylene group having from1 to 3 carbons, Z₂ is an oxygen atom, and E is a glycidyl group.

In the polyorganosiloxane represented by Formula (1) above, X₁ and X₄are preferably epoxy group-containing groups having from 4 to 12 carbonsor alkyl groups having from 1 to 6 carbons among the above, and X₂ ispreferably an epoxy group-containing group having from 4 to 12 carbonsamong the above. More preferably, X₁ and X₄ are alkyl groups having from1 to 6 carbons and X₂ is an epoxy group-containing group having from 4to 12 carbons.

In the polyorganosiloxane represented by Formula (1) above, a grouprepresented by Formula (3) below is preferred as X₃, that is, a groupcontaining from 2 to 20 alkylene glycol repeating units.

In Formula (3) above, t is an integer of 2 to 20, P is an alkyl arylenegroup or an alkylene group having from 2 to 10 carbons, R is a hydrogenatom or a methyl group, and Q is an aryloxy group or an alkoxy grouphaving from 1 to 10 carbons. In Formula (3) above, * indicates a bondposition. Among these, preferably, t is an integer of 2 to 8, P is analkylene group having 3 carbons, R is a hydrogen atom, and Q is amethoxy group.

In the polyorganosiloxane represented by Formula (1) above, m is aninteger of 3 to 200, preferably an integer of 20 to 150, and morepreferably an integer of 30 to 120. Since m is an integer of 3 orgreater, the particular conjugated diene rubber has high affinity withsilica, and as a result, a tire formed from the rubber composition ofthe present technology exhibits excellent low heat build-up.Furthermore, since m is an integer of 200 or less, production of thepolyorganosiloxane is facilitated and the viscosity of the rubbercomposition of the present technology becomes lower.

In the polyorganosiloxane represented by Formula (1) above, n is aninteger of 0 to 200, preferably an integer of 0 to 150, and morepreferably an integer of 0 to 120. Furthermore, in thepolyorganosiloxane represented by Formula (1) above, k is an integer of0 to 200, preferably an integer of 0 to 150, and more preferably aninteger of 0 to 130.

In the polyorganosiloxane represented by Formula (1) above, the totalnumber of m, n, and k is preferably from 3 to 400, more preferably from20 to 300, and particularly preferably from 30 to 250.

Note that, in the polyorganosiloxane represented by Formula (1) above,when the epoxy group in the polyorganosiloxane is reacted with an activeterminal of the conjugated diene-based polymer chain, it is conceivedthat bonding of the carbon atom, located at the part where the epoxygroup ring has been opened, to the active terminal of the conjugateddiene-based polymer chain is formed due to the ring-opening of at leasta part of epoxy group in the polyorganosiloxane. Furthermore, when thealkoxy group in the polyorganosiloxane is reacted with an activeterminal of the conjugated diene-based polymer chain, it is conceivedthat, by eliminating at least a part of the alkoxy group in thepolyorganosiloxane, bonding of the silicon atom, which was bonded to theeliminated alkoxy group, in the polyorganosiloxane to the activeterminal of the conjugated diene-based polymer chain is formed.

The used amount of the polyorganosiloxane (hereinafter, also referred toas “modifying agent”) is an amount such that the ratio of the totalnumber of moles of the epoxy group and the alkoxy group in the modifyingagent to 1 mol of the polymerization initiator used in thepolymerization is preferably in a range from 0.1 to 1, more preferablyin a range from 0.2 to 0.9, and even more preferably in a range from 0.3to 0.8.

In the method of producing the conjugated diene rubber, a part of theactive terminals of the conjugated diene-based polymer chain may beinactivated in the range that does not inhibit the effect of the presenttechnology, by adding a polymerization terminator, a polymerizationterminal modifying agent except the modifying agent described above, anda coupling agent into the polymer system, in addition to modifying theconjugated diene-based polymer chain having an active terminal with themodifying agent described above. That is, in the particular conjugateddiene rubber, a part of the active terminal of the conjugateddiene-based polymer chain may be inactivated by a polymerizationterminator, a polymerization terminal modifying agent except themodifying agent described above, a coupling agent, or the like in therange that does not inhibit the effect of the present technology.

Examples of the polymerization terminal modifying agent and couplingagent used in this case include N-substituted cyclic amides, such asN-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, N-phenyl-2-pyrrolidone,and N-methyl-ε-caprolactam; N-substituted cyclic ureas, such as1,3-dimethylethylene urea and 1,3-diethyl-2-imidazolidinone;N-substituted aminoketones, such as 4,4′-bis(dimethylamino)benzophenoneand 4,4′-bis(diethylamino)benzophenone; aromatic isocyanates, such asdiphenylmethane diisocyanate and 2,4-tolylene diisocyanate;N,N-di-substituted aminoalkylmethacrylamides, such asN,N-dimethylaminopropylmethacrylamide; N-substituted aminoaldehydes,such as 4-N,N-dimethylaminobenzaldehyde; N-substituted carbodiimides,such as dicyclohexylcarbodiimide; Schiff bases, such asN-ethylethylidene imine and N-methylbenzylidene imine; pyridylgroup-containing vinyl compounds, such as 4-vinylpyridine; tintetrachloride; halogenated silicon compounds, such as silicontetrachloride, hexachlorodisilane, bis(trichlorosilyl)methane,1,2-bis(trichlorosilyl)ethane, 1,3-bis(trichlorosilyl)propane,1,4-bis(trichlorosilyl)butane, 1,5-bis(trichlorosilyl)pentane, and1,6-bis(trichlorosilyl)hexane; and the like. Excellent steeringstability is achieved with a tire obtained by using a highly branchedconjugated diene rubber obtained by combined use of a halogenatedsilicon compound having five or more silicon-halogen atom bonds in eachmolecule as a coupling agent. These polymerization terminal modifyingagents and coupling agents may be used alone, or a combination of two ormore types thereof may be used in combination.

When the modifying agent or the like described above is reacted with theactive terminal of the conjugated diene-based polymer chain, it ispreferable to add the modifying agent or the like in a solutioncontaining the conjugated diene-based polymer chain having an activeterminal, and from the perspective of suitably controlling the reaction,it is more preferable to add the modifying agent or the like into apolymerization system by dissolving the modifying agent or the like inan inert solvent. The solution concentration is preferably from 1 to 50mass %.

The timing of adding the modifying agent or the like is not particularlylimited. However, it is desirable to add the modifying agent or the liketo this solution in a state where the polymerization reaction in theconjugated diene-based polymer chain having an active terminal has notbeen completed and the solution containing the conjugated diene-basedpolymer chain having an active terminal contains a monomer. Morespecifically, the modifying agent or the like is preferably added to thesolution in a state where the solution containing the conjugateddiene-based polymer chain having an active terminal contains preferably100 ppm or greater, and more preferably from 300 to 50000 ppm, ofmonomer. By adding the modifying agent or the like in such a manner, itis possible to control the reaction well by suppressing side reactionsbetween the conjugated diene-based polymer chain having an activeterminal and impurities contained in the polymerization system.

As the conditions for reacting the modifying agent or the like describedabove with the active terminal of the conjugated diene-based polymerchain, for example, the temperature is in a range from 0 to 100° C., andpreferably in a range from 30 to 90° C., and the reaction time of eachis in a range from 1 minute to 120 minutes, and preferably in a rangefrom 2 minutes to 60 minutes.

After the modifying agent or the like is reacted with the activeterminal of the conjugated diene-based polymer chain, the unreactedactive terminal is preferably deactivated by adding a polymerizationterminator, for example, an alcohol, such as methanol or isopropanol, orwater.

After the active terminal of the conjugated diene-based polymer chain isdeactivated, anti-aging agents such as phenol-based stabilizers,phosphorus-based stabilizers, or sulfur-based stabilizers, crumblingagents, antiscale agents, and the like are added as desired to thepolymer solution. Thereafter, the polymerization solvent is separatedfrom the polymerization solution by direct drying, steam stripping, orthe like, to recover the resulting particular conjugated diene rubber.Furthermore, before separation of the polymerization solvent from thepolymerization solution, extender oil may be mixed to the polymerizationsolution to recover the particular conjugated diene rubber as anoil-extended rubber.

Examples of the extender oil used when the particular conjugated dienerubber is recovered as an oil-extended rubber include paraffin-based,aromatic, and naphthene-based petroleum-based softeners, plant-basedsofteners, and fatty acids. When the petroleum-based softener is used,the content of the polycyclic aromatics extracted by the method of IP346(determination method of the Institute of Petroleum in the UK) ispreferably less than 3%. When an extender oil is used, the used amountis, for example, from 5 to 100 parts by mass, preferably from 10 to 60parts by mass, and more preferably from 20 to 50 parts by mass, per 100parts by mass of the conjugated diene rubber.

The particular conjugated diene rubber preferably contains from 5 to 40mass %, more preferably from 5 to 30 mass %, and particularly preferablyfrom 10 to 20 mass %, of the structural unit to which three or moreconjugated diene-based polymer chains are bonded and which is formed byreacting the conjugated diene-based polymer chain having an activeterminal and the polyorganosiloxane described above (hereinafter,“structural unit to which three or more conjugated diene-based polymerchains are bonded and which is formed by reacting the conjugateddiene-based polymer chain having an active terminal and thepolyorganosiloxane described above” is simply referred to as “structuralunit to which three or more conjugated diene-based polymer chains arebonded”). When the proportion of the structural unit to which three ormore conjugated diene-based polymer chains are bonded is within therange described above, excellent coagulability and drying propertiesduring production are achieved, and when silica is compounded, a rubbercomposition for tires having even better processability and a tirehaving even better low heat build-up can be provided. Note that theproportion of the structural unit to which three or more conjugateddiene-based polymer chains are bonded relative to the total amount ofthe particular conjugated diene rubber that is obtained in the end (massfraction) is represented as a coupling ratio of three or more branchesof the conjugated diene-based polymer chain. This can be measured by gelpermeation chromatography (based on calibration with polystyrene). Fromthe chart obtained by gel permeation chromatography measurement, theratio of the area of the peak portion having a peak top molecular weightequal to or greater than 2.8 times the peak top molecular weightindicated by the peak of smallest molecular weight to the total elutionarea is taken as the coupling ratio of three or more branches of theconjugated diene-based polymer chain.

The aromatic vinyl unit content of the particular conjugated dienerubber is from 38 to 48 mass %. Of these, from the perspectives ofachieving even better wet performance and excellent low rollingresistance, the content is preferably from 40 to 45 mass %. When thearomatic vinyl unit content is less than 38 mass %, wet performancebecomes insufficient. Furthermore, when the aromatic vinyl unit contentis greater than 48 mass %, low rolling resistance is deteriorated.

The vinyl bond content of the particular conjugated diene rubber is from20 to 35 mass %. Of these, from the perspectives of achieving excellentlow rolling resistance and processability, the content is preferablyfrom 25 to 30 mass %. When the vinyl bond content is less than 20 mass%, low rolling resistance is deteriorated. Furthermore, when the vinylbond content is greater than 35 mass %, the viscosity is increased andprocessability is deteriorated. Note that “vinyl bond content” indicatesthe proportion (mass %) of the vinyl bonds in the conjugated diene unitcontained in the particular conjugated diene rubber.

The weight average molecular weight (Mw) of particular conjugated dienerubber measured by gel permeation chromatography (GPC) based oncalibration with polystyrene is from 500000 to 800000. In particular,the weight average molecular weight (Mw) is preferably from 600000 to700000. When the weight average molecular weight is less than 500000,wear performance is deteriorated. Furthermore, when the weight averagemolecular weight is greater than 800000, processability is deteriorated.

The molecular weight distribution expressed as the ratio (Mw/Mn) of theweight average molecular weight (Mw) to the number average molecularweight (Mn) of the particular conjugated diene rubber is preferably from1.1 to 3.0, more preferably from 1.2 to 2.5, and particularly preferablyfrom 1.2 to 2.2. Note that both Mw and Mn are values measured by GPCbased on calibration with polystyrene.

The Mooney viscosity (ML₁₊₄, 100° C.) of the particular conjugated dienerubber is preferably from 20 to 100, more preferably from 30 to 90, andparticularly preferably from 35 to 80. Note that, when the particularconjugated diene rubber is obtained as an oil-extended rubber, theMooney viscosity of the oil-extended rubber is preferably within therange described above.

In the present technology, the content of the particular conjugateddiene rubber in the diene rubber is from 30 to 80 mass %, preferablyfrom 30 to 70 mass %, and more preferably from 35 to 60 mass %.

When the content of the particular conjugated diene rubber in the dienerubber is less than 30 mass %, wet performance and performance on icyand snowy roads become insufficient.

Note that “content of the particular conjugated diene rubber in thediene rubber” indicates the content (mass %) of the particularconjugated diene rubber relative to the total amount of the dienerubber.

Other Rubber Component

The diene rubber described above may contain another rubber component(other rubber component) besides the butadiene rubber and the particularconjugated diene rubber described above. Such a rubber component is notparticularly limited; however, examples thereof include natural rubber(NR), isoprene rubber (IR), aromatic vinyl-conjugated diene copolymerrubber except the particular conjugated diene rubber,acrylonitrile-butadiene copolymer rubber (NBR), butyl rubber (IIR),halogenated butyl rubber (Br-IIR, Cl-IIR), and chloroprene rubber (CR).Note that examples of the aromatic vinyl-conjugated diene copolymerrubber described above include styrene-butadiene rubber (SBR) except themodified conjugated diene-based rubber, and styrene-isoprene copolymerrubber. Among these, SBR is preferred.

The content of another rubber component in the diene rubber is notparticularly limited; however, the content is preferably from 10 to 30mass %. Note that “content of another rubber component in the dienerubber” indicates the content (mass %) of another rubber componentrelative to the total amount of the diene rubber.

In the present technology, the average glass transition temperature (Tg)of such a diene rubber is from −65 to −45° C., and preferably from −60to −50° C. Note that the average Tg of the diene rubber is calculated bymultiplying the Tg of component of each rubber by the number of mass %of each rubber component, and then summing up the obtained values.Furthermore, Tg of each rubber is calculated by the midpoint methodbased on the measurement using a differential scanning calorimetry (DSC)at a rate of temperature increase of 20° C./min.

Silica

The silica contained in the rubber composition of the present technologyis not particularly limited, and any conventionally known silica that iscompounded into a rubber composition in applications such as tires canbe used.

Specific examples of the silica include fumed silica, calcined silica,precipitated silica, pulverized silica, molten silica, and colloidalsilica. One type of these may be used alone, or two or more types may beused in combination.

In the present technology, from the perspective of enhancing performanceon icy and snowy roads, the silica preferably contains 50 mass % orgreater, and more preferably from 50 to 100 mass %, of silica (X) havinga CTAB adsorption specific surface area of 80 to 130 m²/g. Note that“containing 100 mass %” indicates that only the silica (X) is containedas the silica described above.

Note that the CTAB adsorption specific surface area is a value of theamount of n-hexadecyltrimethylammonium bromide adsorbed to the surfaceof silica measured in accordance with JIS (Japanese Industrial Standard)K6217-3:2001 “Part 3: Method for determining specific surface area—CTABadsorption method.”

In the rubber composition of the present technology, the content of thesilica (when the silica (X) described above and another silica are usedin combination, the total amount of these) is from 90 to 150 parts bymass, preferably from 95 to 145 parts by mass, and more preferably from100 to 140 parts by mass, per 100 parts by mass of the diene rubberdescribed above.

When the content of the silica is less than 90 parts by mass, wetperformance becomes insufficient, and when the content is greater than150 parts by mass, performance on icy and snowy roads becomesinsufficient.

Alkyltrialkoxysilane

The alkyltrialkoxysilane contained in the rubber composition of thepresent technology is an alkyltrialkoxysilane represented by Formula (I)below.

In Formula (I), R¹¹ represents an alkyl group having from 1 to 20carbons, and R¹² each independently represents a methyl group or anethyl group.

Note that specific examples of the alkyl group having from 1 to 20carbons of R¹¹ include a methyl group, an ethyl group, a propyl group, abutyl group, a pentyl group, a hexyl group, a heptyl group, an octylgroup, a nonyl group, a decyl group, an undecyl group, and a dodecylgroup.

Among these, from the perspective of miscibility with the rubbercomponent described above, the number of carbons of R¹¹ is preferably 7or more, and specifically, an octyl group and a nonyl group are morepreferred.

In the rubber composition of the present technology, the content of thealkyltrialkoxysilane is from 0.1 to 8 mass %, and preferably from 1 to 7mass %, relative to the content of the silica described above.

When the content of the alkyltrialkoxysilane is less than 0.1 mass % orgreater than 8 mass %, wet performance and performance on icy and snowyroads become insufficient.

Optional Components

The rubber composition of the present technology may further containanother component (optional component) as necessary within the scopethat does not inhibit the effect or purpose thereof.

Examples of the optional component include various additives, such asfillers except silica (e.g. carbon black), silane coupling agents,aromatic modified terpene resins, zinc oxide (zinc white), stearic acid,anti-aging agents, waxes, processing aids, oils, liquid polymers,thermosetting resins, vulcanizing agents (e.g. sulfur), andvulcanization accelerators.

Among these, the silane coupling agent and the aromatic modified terpeneresin described below are preferably contained.

Silane Coupling Agent

The silane coupling agent is not particularly limited, and anyconventionally known silane coupling agent that is blended in rubbercompositions for applications such as tires can be used.

Specific examples of the silane coupling agent includebis(3-triethoxysilylpropyl)tetrasulfide,bis(3-triethoxysilylpropyl)trisulfide,bis(3-triethoxysilylpropyl)disulfide,bis(2-triethoxysilylethyl)tetrasulfide,bis(3-trimethoxysilylpropyl)tetrasulfide,bis(2-trimethoxysilylethyl)tetrasulfide,3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane,2-mercaptoethyltrimethoxysilane, 2-mercaptoethyltriethoxysilane,3-trimethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,3-triethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,2-triethoxysilylethyl-N,N-dimethylthiocarbamoyl tetrasulfide,3-trimethoxysilylpropyl benzothiazole tetrasulfide,3-triethoxysilylpropyl benzothiazole tetrasulfide,3-triethoxysilylpropyl methacrylate monosulfide, 3-trimethoxysilylpropylmethacrylate monosulfide, bis(3-diethoxymethylsilylpropyl)tetrasulfide,dimethoxymethylsilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide, anddimethoxymethylsilylpropyl benzothiazole tetrasulfide. One type of thesemay be used alone, or two or more types may be used in combination. Inaddition, one or two or more types of these may be oligomerized inadvanced and used.

Furthermore, specific examples of the silane coupling agent except thoselisted above include mercapto-based silane coupling agents, such asγ-mercaptopropyltriethoxysilane and3-[ethoxybis(3,6,9,12,15-pentaoxaoctacosan-1-yloxy)silyl]-1-propanethiol;thiocarboxylate-based silane coupling agents, such as3-octanoylthiopropyltriethoxysilane; and thiocyanate-based silanecoupling agents, such as 3-thiocyanatepropyltriethoxysilane. One type ofthese may be used alone, or two or more types may be used incombination. In addition, one or two or more types of these may beoligomerized in advanced and used.

Among these, from the perspective of effect of enhancing reinforcingproperties, bis(3-triethoxysilylpropyl)tetrasulfide and/orbis(3-triethoxysilylpropyl)disulfide is preferably used. Specificexamples thereof include Si69 (bis(3-triethoxysilylpropyl)tetrasulfide,manufactured by Evonik Degussa), and Si75(bis(3-triethoxysilylpropyl)disulfide, manufactured by Evonik Degussa).

In the rubber composition of the present technology, in the case wherethe silane coupling agent described above is contained, the contentthereof is preferably from 3 to 20 mass %, and more preferably from 4 to15 mass %, relative to the content of the silica described above.

In the present technology, since wet performance and performance on icyand snowy roads are further enhanced, the silane coupling agent and thesilica particularly preferably satisfy Equation (A) below.

Note that in Equation (A) below “content of silane coupling agent (g)”and “content of silica (g)” each indicate a content (g) relative to 100(g) of the diene rubber.[content of silane coupling agent (g)×100/{content of silica (g)×CTABadsorption specific surface area (m²/g)}]×100=3.5 to 7.5  Equation (A):Aromatic Modified Terpene Resin

The rubber composition of the present technology preferably furthercontains an aromatic modified terpene resin from the perspective offurther enhancing wet performance.

The softening point of the aromatic modified terpene resin is notparticularly limited; however, the softening point is preferably from100 to 150° C., and more preferably from 100 to 130° C.

Note that the softening point is a Vicat softening point measured inaccordance with JIS K7206:1999.

In the rubber composition of the present technology, in the case wherethe aromatic modified terpene resin is contained, the content thereof isnot particularly limited; however, the content is preferably from 1 to30 parts by mass, and more preferably from 2 to 20 parts by mass, per100 parts by mass of the diene rubber described above.

Method of Preparing Rubber Composition for Tires

The method of producing the rubber composition of the present technologyis not particularly limited, and specific examples thereof include amethod whereby each of the above-mentioned components is kneaded using apublicly known method and device (e.g. Banbury mixer, kneader, androller). When the rubber composition of the present technology containssulfur or a vulcanization accelerator, it is preferable to blend thecomponents other than the sulfur and the vulcanization accelerator firstat a high temperature (preferably from 100° C. to 160° C.), cool themixture, and then blend the sulfur and the vulcanization acceleratorthereto.

In addition, the rubber composition of the present technology can bevulcanized or crosslinked under conventional, publicly known vulcanizingor crosslinking conditions.

Pneumatic Tire

The pneumatic tire of the present technology is a pneumatic tireproduced by using the rubber composition of the present technologydescribed above. In particular, the pneumatic tire of the presenttechnology is preferably a pneumatic tire having the rubber compositionof the present technology in a cap tread, and is more preferably used asa pneumatic tire for winter.

FIG. 1 is a partial cross-sectional schematic view of a tire thatrepresents one embodiment of the pneumatic tire of the presenttechnology, but the pneumatic tire of the present technology is notlimited to the embodiment illustrated in FIG. 1.

In FIG. 1, reference sign 1 indicates a bead portion, reference sign 2indicates a sidewall portion, and reference sign 3 indicates a tiretread portion.

In addition, a carcass layer 4, in which a fiber cord is embedded, ismounted between a left-right pair of bead portions 1, and ends of thecarcass layer 4 are wound by being folded around bead cores 5 and a beadfiller 6 from an inner side to an outer side of the tire.

In the tire tread portion 3, a belt layer 7 is provided along the entirecircumference of the tire on the outer side of the carcass layer 4.

Additionally, rim cushions 8 are provided in parts of the bead portions1 that are in contact with a rim.

Note that the tire tread portion 3 is formed by the rubber compositionof the present technology described above.

The pneumatic tire of the present technology can be produced, forexample, in accordance with a conventionally known method. In additionto ordinary air or air with an adjusted oxygen partial pressure, inertgases such as nitrogen, argon, and helium can be used as the gas withwhich the tire is filled.

EXAMPLES

Hereinafter, the present technology will be further described in detailwith reference to examples; however, the present technology is notlimited thereto.

Preparation of Particular Conjugated Diene Rubber

In a nitrogen-purged 100 mL ampoule bottle, cyclohexane (35 g) andtetramethylethylenediamine (1.4 mmol) were added, and thenn-butyllithium (4.3 mmol) was added. Thereafter, isoprene (21.6 g) andstyrene (3.1 g) were gradually added and reacted in the ampoule bottleat 50° C. for 120 minutes to obtain a polymer block A having an activeterminal. For this polymer block A, the weight average molecular weight,molecular weight distribution, aromatic vinyl unit content, isopreneunit content, and 1,4-bond content were measured. These measurementresults are shown in Table 1.

Thereafter, in an autoclave equipped with a stirrer, cyclohexane (4000g), 1,3-butadiene (474.0 g), and styrene (126.0 g) were charged in anitrogen atmosphere, and then the total amount of the polymer block Ahaving an active terminal obtained as described above was added toinitiate polymerization at 50° C. After checking that the polymerconversion rate was in a range from 95% to 100%, the polyorganosiloxaneA represented by Formula (4) below was added in a state of a xylenesolution with a concentration of 20 mass % in a manner that the contentof the epoxy group was 1.42 mmol (equivalent to 0.33 times the number ofmoles of the used n-butyllithium), and reacted for 30 minutes. Afterthat, methanol in an amount equivalent to twice the number of moles ofthe used n-butyllithium was added as a polymerization terminator, and asolution containing a particular conjugated diene rubber was obtained.To this solution, a small amount of anti-aging agent (IRGANOX 1520,manufactured by BASF) and, as an extension oil, 25 parts by mass ofFukkoru Eramikku 30 (manufactured by Nippon Oil Corporation) per 100parts by mass of the particular conjugated diene rubber were added.Then, a solid rubber was recovered using a steam stripping process. Bydehydrating the obtained solid rubber using a roll and drying it in adryer, a solid particular conjugated diene rubber was obtained.

In Formula (4) above, each of X₁, X₄, R₁ to R₃, and R₅ to R₈ is a methylgroup. In Formula (4) above, m is 80, and k is 120. In Formula (4)above, X₂ is a group represented by Formula (5) below (in the formula, *indicates a bond position).

For the obtained particular conjugated diene rubber, the weight averagemolecular weight, molecular weight distribution, coupling ratio of threeor more branches, aromatic vinyl unit content, vinyl bond content, andMooney viscosity were measured. The measurement results are shown inTable 2. The measurement methods are as follows.

Weight Average Molecular Weight, Molecular Weight Distribution, andCoupling Ratio of Three or More Branches

The weight average molecular weight, molecular weight distribution, andcoupling ratio of three or more branches (proportion (mass %) of“structural unit to which three or more conjugated diene-based polymerchains are bonded” relative to the amount of the particular conjugateddiene rubber) were determined using a chart obtained by gel permeationchromatography based on molecular weight based on calibration withpolystyrene. The specific gel permeation chromatography measurementmethod is as follows.

-   -   Measurement instrument: HLC-8020 (manufactured by Tosoh Corp.)    -   Column: GMH-HR-H (manufactured by Tosoh Corp.), two connected in        series    -   Detector: Differential refractometer RI-8020 (manufactured by        Tosoh Corp.)    -   Eluent: Tetrahydrofuran    -   Column temperature: 40° C.

Here, the coupling ratio of three or more branches is the ratio (s2/s1)of the area (s2) of the peak portion having a peak top molecular weightequal to or greater than 2.8 times the peak top molecular weightindicated by the peak of smallest molecular weight to the total elutionarea (s1).

Aromatic Vinyl Unit Content and Vinyl Bond Content

The aromatic vinyl unit content and the vinyl bond content were measuredby ¹H-NMR.

Mooney Viscosity

The Mooney viscosity (ML₁₊₄, 100° C.) was measured in accordance withJIS K6300-1:2013.

TABLE 1 Polymer block A Weight average molecular weight 8700 Molecularweight distribution 1.10 (Mw/Mn) Aromatic vinyl unit content (mass %)12.6 Isoprene unit content (mass %) 87.4 1,4-Bond content (mass %) 58.0

TABLE 2 Particular conjugated diene rubber Weight average molecularweight 640000 Molecular weight distribution 1.65 (Mw/Mn) Coupling ratioof three or more branches 12.5 (mass %) Aromatic vinyl unit content(mass %) 42.6 Vinyl bond content (mass %) 29.5 Mooney viscosity (ML₁₊₄,100° C.) 58Production of Comparative Conjugated Diene Rubber

To a nitrogen-purged autoclave reaction vessel having an internalcapacity of 10 L, 4533 g of cyclohexane, 338.9 g (3.254 mol) of styrene,468.0 g (8.652 mol) of butadiene, 20.0 g (0.294 mol) of isoprene, and0.189 mL (1.271 mmol) of N,N,N′,N′-tetramethylethylenediamine werecharged. Then, agitation was begun. After the temperature of the contentin the reaction vessel was adjusted to 50° C., 5.061 mL (7.945 mmol) ofn-butyllithium was added. After the polymer conversion rate reachedapproximately 100%, 12.0 g of isoprene was added and the mixture wasreacted for 5 minutes. Then, 0.281 g (0.318 mmol) of a toluene solutioncontaining 40 wt. % of 1,6-bis(trichlorosilyl)hexane was added and themixture was reacted for 30 minutes. Furthermore, the polyorganosiloxaneA represented by Formula (4) above was added in a state of a xylenesolution with a concentration of 20 mass % in a manner that the contentof the epoxy group was 1.00 mmol (equivalent to 0.13 times the number ofmoles of the used n-butyllithium), and reacted for 30 minutes. Thereto,0.5 mL of methanol was added and agitated for 30 minutes to obtain asolution containing conjugated diene rubber. To the obtained solution, asmall amount of anti-aging agent (IRGANOX 1520, manufactured by BASF)and, as an extension oil, 25 parts by mass of Fukkoru Eramikku 30(manufactured by Nippon Oil Corporation) per 100 parts by mass of theconjugated diene rubber were added. Then, a solid rubber was recoveredusing a steam stripping process. By dehydrating the obtained solidrubber using a roll and drying it in a dryer, a solid conjugated dienerubber was obtained. The obtained conjugated diene rubber was used as acomparative conjugated diene rubber.

Preparation of Rubber Composition for Tires

The components shown in Table 3 below were blended in the proportions(part by mass) shown in Table 3 below.

Specifically, a master batch was obtained by mixing the components shownin Table 3, excluding the sulfur and the vulcanization accelerator for 5minutes in a 1.7 L closed-type Banbury mixer heated to a temperaturenear 150° C., and then discharging the mixture and cooling it to roomtemperature. The sulfur and vulcanization accelerator were then mixedinto the resulting master batch using the Banbury mixer described aboveso as to obtain a rubber composition for tires.

Note that, in Table 3, the numbers of part by mass of the particularconjugated diene rubber, the comparative conjugated diene rubber, andthe SBR 1 are net amounts of rubbers excluding the extender oils (unit:part by mass).

Evaluation

The following evaluations were performed for the obtained rubbercomposition for tires.

Wet Performance

A vulcanized rubber sheet was produced by press-vulcanizing the obtained(unvulcanized) rubber composition for tires for 20 minutes at 160° C. ina mold (15 cm×15 cm×0.2 cm).

The value of tan δ (0° C.) was measured for the produced vulcanizedrubber sheet with an elongation deformation strain of 10%±2%, afrequency of 20 Hz, and a temperature of 0° C. using a viscoelasticspectrometer (manufactured by Toyo Seiki Seisaku-sho, Ltd.) inaccordance with JIS K6394:2007.

The results are shown in Table 3 (row of “wet performance” in Table 3).The result was expressed as an index with the value of tan δ (0° C.) ofStandard Example expressed as an index of 100. A larger index indicatesa larger tan δ (0° C.) value, which in turn indicates excellent wetperformance when a tire is produced.

Performance on Icy and Snowy Roads

Tires produced using the obtained rubber composition for tires in theirtread portions and having a tire size of 205/55R16 were mounted on anABS-equipped vehicle having an engine displacement of 2000 cc. Bothfront tires and rear tires were inflated to an air pressure of 220 kPa.Braking/stopping distance was measured on an icy and snowy road surfaceat a speed of 40 km. The result was expressed as an index with thebraking/stopping distance of Standard Example expressed as an index of100. A larger index indicates a shorter braking distance and superiorperformance on icy and snowy roads.

TABLE 3 Standard Comparative Example Example 1 2 3 4 5 6 7 8 Particular10 40 40 40 40 40 40 40 conjugated diene rubber Comparative 40 30conjugated diene rubber SBR 1 20 20 20 20 40 10 20 20 SBR 2 40 BR 40 4040 40 20 50 40 40 10 Silica 1 100 100 80 160 100 100 100 100 100 SilicaX-1 Carbon black 5 5 5 5 5 5 5 5 5 Silane coupling 8.0 8.0 6.4 12.8 8.08.0 8.0 8.0 8.0 agent (value in (5.0) (5.0) (5.0) (5.0) (5.0) (5.0)(5.0) (5.0) (5.0) Equation A) Alkylsilane 1 2 2 2 2 2 2 10 2 Alkylsilane2 Alkylsilane 3 2 Aromatic modified terpene resin Zinc oxide 4 4 4 4 4 44 4 4 Stearic acid 2 2 2 2 2 2 2 2 2 Anti-aging agent 2 2 2 2 2 2 2 2 2Wax 2 2 2 2 2 2 2 2 2 Oil 35 35 28 55 28 39 35 35 43 Sulfur 1.5 1.5 1.51.5 1.5 1.5 1.5 1.5 1.5 Vulcanization 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.51.5 accelerator 1 Vulcanization 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3accelerator 2 Average glass −59 −59 −59 −59 −43 −67 −59 −59 −57transition temperature of diene rubber (° C.) Total oil amount 53 53 4673 53 53 53 53 53 Wet performance 100 102 90 117 112 88 101 98 108Performance on 100 101 105 88 92 104 102 98 95 icy and snowy roadsStandard Example Example 1 2 3 4 5 6 7 8 9 Particular 40 40 40 60 40 4040 40 40 conjugated diene rubber Comparative 40 conjugated diene rubberSBR 1 20 20 20 20 20 20 20 20 20 SBR 2 BR 40 40 40 40 40 40 40 40 40 40Silica 1 100 100 50 100 100 100 100 100 100 Silica X-1 50 100 Carbonblack 5 5 5 5 5 5 5 5 5 5 Silane coupling 8.0 8.0 6.6 5.3 8.0 8.0 8.05.0 13.0 8.0 agent (value in (5.0) (5.0) (5.0) (5.0) (5.0) (5.0) (5.0)(3.1) (8.1) (5.0) Equation A) Alkylsilane 1 2 2 2 2 2 7 2 2 2Alkylsilane 2 2 Alkylsilane 3 Aromatic modified 10 terpene resin Zincoxide 4 4 4 4 4 4 4 4 4 4 Stearic acid 2 2 2 2 2 2 2 2 2 2 Anti-agingagent 2 2 2 2 2 2 2 2 2 2 Wax 2 2 2 2 2 2 2 2 2 2 Oil 35 35 35 35 38 3535 35 35 25 Sulfur 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Vulcanization1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 accelerator 1 Vulcanization 0.30.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 accelerator 2 Average glass −59 −59−59 −59 −59 −59 −59 −59 −59 −59 transition temperature of diene rubber(° C.) Total oil amount 53 53 53 53 53 53 53 53 53 43 Wet performance100 111 109 108 117 114 108 105 108 119 Performance on 100 106 110 116109 110 104 103 104 105 icy and snowy roads

In Table 3, details of the components are as follows.

-   -   Particular conjugated diene rubber: Particular conjugated diene        rubber produced as described above (containing 25 parts by mass        of extender oil per 100 parts by mass of the rubber)    -   Comparative conjugated diene rubber: Comparative conjugated        diene rubber produced as described above (containing 25 parts by        mass of extender oil per 100 parts by mass of the rubber)        (aromatic vinyl unit content: 42 mass %; vinyl bond content: 32        mass %; Tg: −25° C.; Mw: 750000 (measurement methods were as        described above))    -   SBR 1: BUNA VSL 5025-2 (oil extended product containing 37.5        parts by mass of extender oil per 100 parts by mass of the        rubber content; manufactured by Lanxess)    -   SBR 2: Tufdene 1000R (manufactured by Asahi Kasei Corporation)    -   BR: Nipol BR 1220 (manufactured by Zeon Corporation)    -   Silica 1: Zeosil 1165MP (CTAB adsorption specific surface area:        159 m²/g, manufactured by Solvay)    -   Silica X-1: Zeosil 1115MP (CTAB adsorption specific surface        area: 110 m²/g, manufactured by Rhodia)    -   Carbon black: N339 (manufactured by Cabot Japan K.K.)    -   Silane coupling agent: Bis(3-triethoxysilylpropyl)tetrasulfide        (Si69, manufactured by Evonik Degussa)    -   Alkylsilane 1: Octyltriethoxysilane (KBE-3083, manufactured by        Shin-Etsu Chemical Co., Ltd.)    -   Alkylsilane 2: Decyltrimethoxysilane (KBM-3103, manufactured by        Shin-Etsu Chemical Co., Ltd.)    -   Alkylsilane 3: Dimethyldiethoxysilane (KBE-22, manufactured by        Shin-Etsu Chemical Co., Ltd.)    -   Aromatic modified terpene resin: YS RESIN TO-125 (softening        point: 125±5° C.; manufactured by Yasuhara Chemical Co., Ltd.)    -   Zinc oxide: Zinc Oxide #3 (manufactured by Seido Chemical        Industry Co., Ltd.)    -   Stearic acid: Beads stearic acid YR (manufactured by Nippon Oil        & Fats Co., Ltd.)    -   Anti-aging agent: 6PPD (manufactured by Flexsys)    -   Wax: Paraffin wax (manufactured by Ouchi Shinko Chemical        Industrial Co., Ltd.)    -   Oil: Extract No. 4S (manufactured by Showa Shell Sekiyu K.K.)    -   Sulfur: Oil-treated sulfur (manufactured by Hosoi Chemical        Industry Co., Ltd.)    -   Vulcanization accelerator 1: Sanceller CM-G (manufactured by        Sanshin Chemical Industry Co., Ltd.)    -   Vulcanization accelerator 2: Perkacit DPG grs (manufactured by        Flexsys)

As is clear from Table 3, it was found that the rubber composition ofComparative Example 1, in which the compounded amount of the particularconjugated diene rubber was outside the predetermined range, hardlyenhanced the wet performance and the performance on icy and snowy roadseven when compared with Standard Example (Comparative Example 1).

Furthermore, it was found that the rubber composition of ComparativeExample 2, in which the compounded amount of the silica was less thanthe predetermined range, resulted in poor wet performance, and therubber composition of Comparative Example 3, in which the compoundedamount of the silica was greater than the predetermined range, resultedin poor performance on icy and snowy roads (Comparative Examples 2 and3).

Furthermore, it was found that the rubber composition of ComparativeExample 4, in which the average glass transition temperature of thediene rubber was higher than the predetermined temperature range,resulted in poor performance on icy and snowy roads, and the rubbercomposition of Comparative Example 5, in which the average glasstransition temperature of the diene rubber was lower than thepredetermined temperature range, resulted in poor wet performance(Comparative Examples 4 and 5).

Furthermore, it was found that rubber compositions of ComparativeExample 6, in which the compounded amount of the alkyltrialkoxysilanewas outside the predetermined range, and of Comparative Example 7, inwhich dimethyldiethoxysilane was compounded, hardly enhanced the wetperformance and the performance on icy and snowy roads even whencompared with Standard Example (Comparative Examples 6 and 7).

Furthermore, the rubber composition of Comparative Example 8, in whichthe compounded amount of the butadiene rubber in the diene rubber wasless than the predetermined range, resulted in poor performance on icyand snowy roads (Comparative Example 8).

On the other hand, the rubber compositions of Examples 1 to 9, in whichthe predetermined amounts of the particular conjugated diene rubber andthe butadiene rubber were compounded and the predetermined amounts ofthe silica and the alkyltrialkoxysilane were compounded, achievedexcellent wet performance and performance on icy and snowy roads.

In particular, from the comparison of Examples 1, 7, and 8, it was foundthat the wet performance and the performance on icy and snowy roads werefurther enhanced due to the compounded silane coupling agent and silicasatisfying Equation (A) above.

Furthermore, from the comparison of Examples 1 to 3, it was found thatthe performance on icy and snowy roads was further enhanced when 50 mass% or greater of silica having a CTAB adsorption specific surface area of80 to 130 m²/g was compounded.

Furthermore, from the comparison of Examples 1 and 9, it was found thatthe wet performance was further enhanced when the aromatic modifiedterpene resin having a softening point of 100 to 150° C. was compounded.

The invention claimed is:
 1. A rubber composition for tires comprising adiene rubber, silica, an alkyltrialkoxysilane represented by Formula(I), and a silane coupling agent; the diene rubber containing abutadiene rubber and a particular conjugated diene rubber, a content ofthe butadiene rubber in the diene rubber being 20 mass % or greater anda content of the particular conjugated diene rubber in the diene rubberbeing from 30 to 80 mass %; an average glass transition temperature ofthe diene rubber being from −65 to −45° C.; a content of the silicabeing from 90 to 150 parts by mass per 100 parts by mass of the dienerubber; a content of the alkyltrialkoxysilane being from 0.1 to 8 mass %relative to the content of the silica; the silane coupling agent and thesilica satisfying a relationship: [content of silane coupling agent(g)×100/{content of silica (g)/CTAB adsorption specific surface area(m2/g)}]/100=3.5 to 7.5;

wherein, R¹¹ represents an alkyl group having from 1 to 20 carbons, andR¹² each independently represents a methyl group or an ethyl group; theparticular conjugated diene rubber being a conjugated diene rubberproduced by a method of producing a conjugated diene rubber, the methodcomprising steps A, B, and C below in this order; the particularconjugated diene rubber having an aromatic vinyl unit content of 38 to48 mass %, a vinyl bond content of 20 to 35 mass %, and a weight averagemolecular weight of 500000 to 800000; Step A: a step of forming apolymer block A having an active terminal, the polymer block A having anisoprene unit content of 80 to 95 mass %, an aromatic vinyl unit contentof 5 to 20 mass %, and a weight average molecular weight of 500 to15000, by polymerizing a monomer mixture containing isoprene and anaromatic vinyl; Step B: a step of obtaining a conjugated diene-basedpolymer chain having an active terminal, the conjugated diene-basedpolymer chain having the polymer block A and a polymer block B, byforming the polymer block B having an active terminal by mixing thepolymer block A with a monomer mixture containing 1,3-butadiene and anaromatic vinyl to continue polymerization reaction to form the polymerblock B in series with the polymer block A; and Step C: a step ofreacting a polyorganosiloxane represented by Formula (1) with the activeterminal of the conjugated diene-based polymer chain;

wherein, R₁ to R₈ are the same or different and are each an alkyl grouphaving from 1 to 6 carbons or an aryl group having from 6 to 12 carbons;X₁ and X₄ are the same or different and are groups selected from thegroup consisting of alkyl groups having from 1 to 6 carbons, aryl groupshaving from 6 to 12 carbons, alkoxy groups having from 1 to 5 carbons,and groups having an epoxy group and from 4 to 12 carbons; X₂ is analkoxy group having from 1 to 5 carbons or a group having an epoxy groupand from 4 to 12 carbons, and a plurality of the X₂ moieties are thesame or different; X₃ is a group having from 2 to 20 alkylene glycolrepeating units, and when a plurality of the X₃ moieties exists, the X₃moieties are the same or different; and m is an integer from 3 to 200, nis an integer from 0 to 200, and k is an integer from 0 to
 200. 2. Therubber composition for tires according to claim 1, wherein the silicacontains 50 mass % or greater of silica (X) having a CTAB adsorptionspecific surface area of 80 to 130 m²/g.
 3. The rubber composition fortires according to claim 1, further comprising an aromatic modifiedterpene resin having a softening point of 100 to 150° C., a content ofthe aromatic modified terpene resin being from 3 to 20 parts by mass per100 parts by mass of the diene rubber.
 4. A pneumatic tire having therubber composition for tires described in claim 1 in a cap tread.
 5. Thepneumatic tire according to claim 4, the pneumatic tire being used for awinter tire.
 6. The rubber composition for tires according to claim 2,further comprising an aromatic modified terpene resin having a softeningpoint of 100 to 150° C., a content of the aromatic modified terpeneresin being from 3 to 20 parts by mass per 100 parts by mass of thediene rubber.
 7. A pneumatic tire having the rubber composition fortires described in claim 2 in a cap tread.
 8. A pneumatic tire havingthe rubber composition for tires described in claim 3 in a cap tread. 9.A pneumatic tire having the rubber composition for tires described inclaim 6 in a cap tread.